1. Field of the Invention
The present invention relates to methods and apparatus of geophysical exploration and more particularly to such methods and apparatus wherein a plurality of explosive charges located at spaced intervals within a bore hole are sequentially detonated.
2. Description of the Related Art
In one method of seismic exploration, explosive charges are detonated in an underground formation and a recording of the pressure waves generated in the formation is made. Those knowledgeable in the interpretation of such pressure wave recordings can deduce information about the formation such as whether structures capable of trapping oil or gas are present.
One technique used for generating such pressure waves involves drilling a bore hole and distributing explosive charges at spaced intervals therein. The explosive charges are caused to detonate sequentially down the bore hole thereby creating a pressure wave which travels down the bore hole and which is reinforced by each successive explosion.
There are several known methods and apparatus for causing such sequential detonation of charges in a bore hole. In one such method and apparatus, each charge in the bore hole, except for the uppermost charge, is associated with a detonating circuit which includes a capacitor, an electro-acoustic sensor which generates a voltage in response to a pressure wave and a timer.
In operation, each of the detonating circuits is connected in parallel to a pair of wires which extend to the surface of the bore hole. Each capacitor is charged by applying a direct current voltage to the wires at the surface. The uppermost charge in the bore hole is detonated by a blaster in a conventional manner. The pressure wave so created strikes each sensor which in turn activates the associated timer. When a preselected timing period has passed, a circuit connection is made which causes the capacitor to discharge into the detonator thereby exploding the charge.
It would be desirable to provide a method and apparatus for sequentially detonating a plurality of charges in a bore hole in which the sensitivity of the sensor and its associated circuitry is adjustable. Premature detonation of the charges in response to background noise such as nearby drilling, transportation or construction may occur if the sensor and associated firing circuit is too sensitive. It would also be desirable to provide a pre-firing indication that the sensor will cause the detonator to explode in response to background noise. In the prior art, the only way the operator has of knowing whether or not background noise will cause premature detonation is to install the charges, detonators and related firing circuits in a bore hole, charge the capacitors in each detonating circuit and see whether or not one or more of the charges explodes. Such premature detonation prevents proper operation of the system, i.e., interferes with or prevents creation of a downwardly-directed pressure wave which is reinforced by successive explosions in the bore hole.
It would also be desirable to provide such a method and apparatus which is armed only very briefly just prior to explosion of the top charge in the bore hole in order to further minimize the possibility of premature detonation.
Sometimes, a pressure wave generated by a charge detonated at the top of the bore hole travels to the bottom thereof without detonating each of the charges distributed in the bore hole. Pressure waves generated by detonation of the distributed charges can cause detonation of the unexploded charges two to three seconds after the initial pressure wave. Such late explosions destroy the value of the seismic recordings and are thus undesirable. It would be advantageous for the firing circuits to be armed for a predetermined time which would typically fall between the 0.1 and 0.5 second range. Thus, in the event that a charge is not exploded by the initial pressure wave, subsequent pressure waves generated by reflections and/or detonation of distributed charges do not cause late detonation of unexploded charges.
Still, it may be undesirable to leave unexploded charges in the ground. Thus, it would be advantageous for any unexploded charges remaining in the ground to be first disarmed as described above to permit seismic recording without late detonation and, after completion of the seismic recording, to be rearmed and detonated so as not to leave unexploded charges in the ground.
In the event of equipment problems or unexpected sources of noise, such as nearby air or ground traffic, it would be desirable to remove the capacitor charging voltage of the surface without risk of charge detonation. When the prior art capacitor charging voltage is removed, the operator is left with a string of charges in the bore hole prepared to fire in response to a pressure wave.
As mentioned, prior art methods and apparatus include a pair of wires to which a direct current voltage is applied at the surface for charging the capacitor in each detonating circuit. It is important that the correct polarity voltage is applied to the wires to avoid premature detonation or damage to the circuit components which could leave a string of live charges in the hole without means for detonating them. It would thus be desirable if the voltage could be applied at the surface and the detonating circuits could be connected to the charging wires without regard for polarity.